1. Field of the Invention
The present invention relates generally to an apparatus and a method for performing a handover of a terminal in a wireless communication system, and in particular, to an apparatus and a method for supporting fast resumption of downlink data reception of a terminal through fast update of a Connection Identification (CID) during a handover in an Institute of Electrical and Electronics Engineers (IEEE) 802.16e system.
2. Description of the Related Art
IEEE 802.16e supports an inter-cell handover in order to guarantee mobility of a terminal. FIGS. 1 and 2 are views illustrating a conventional handover procedure supported by IEEE 802.16e.
First, the conventional handover procedure is described below with reference to FIG. 1. When it is determined that a handover is required, a terminal 100 transmits a Mobile_Mobile Station Handover-Request (MOB_MSHO-REQ) message for a handover request to a serving base station 120 which the terminal 100 is currently accessing (step 101). At this point, the serving base station 120 receives the message and transmits a HandOver (HO) Request message for a handover request to target base stations 130 and 140 which the terminal 100 desires to access (steps 102 and 103). Each of the target base stations 130 and 140 determines whether to allow the handover of the terminal 100, and transmits a HO Response message for informing a determination result to the serving base station 120 (steps 104 and 105). At this point, the serving base station 120 consolidates HO Response messages from the target base stations 130 and 140, and transmits a Mobile_Base Station Handover-Response (MOB_BSHO-RSP) message to the terminal 100 (step 106). The terminal 100 obtains information regarding the target base stations that have allowed the handover through the message, determines a target base station on which the handover is to be performed, informs the serving base station 120 of a determination result through a Mobile_Handover-Indication (MOB_HO-IND) message, and performs the handover (step 107). At this point, the serving base station 120 transmits a HO confirm message to the target base station 130 on which the handover is to be performed, to inform the target base station 130 that the terminal 100 is scheduled to actually perform the handover and access the target base station 130 (step 108).
After that, the terminal 100 which is to perform the handover performs a handover ranging on the target base station. Examples of a conventional method for performing the handover ranging include a method using a ranging code and a method using a Fast_Ranging_IE. FIG. 1 corresponds to the method using the ranging code. According to the method using the ranging code, the terminal 100 transmits a ranging code arbitrarily selected from a handover ranging code region to the target base station 130 to perform the handover ranging (step 109). Here, the ranging code is a competitiveness-based arbitrary code for synchronization in a physical layer and informing an entry of the terminal 100. At this point, the target base station 130 transmits a physical channel correction value by transmitting a Ranging-Response (RNG-RSP) message for the ranging code to the terminal 100, and informs resource assignment for transmission of Ranging-Request (RNG-REQ) message by the terminal 100 through UpLink-MAP (UL-MAP) (step 110). After that, the terminal 100 transmits an RNG-REQ message to the target base station 130 using an assigned resource (step 111), and the target base station 130 transmits an RNG-RSP message including CID update information to the terminal 100 in response thereto (step 112). Through this procedure, data transmission/reception between the terminal 100 and the target base station 130 can be performed (step 113).
FIG. 2 corresponds to the method for performing a handover ranging using the Fast_Ranging_IE. Here, steps before the handover ranging is performed are the same as those of FIG. 1. That is, steps 201 to 208 are the same as steps 101 to 108 of FIG. 1. Therefore, only steps of performing the handover ranging will be described below. A terminal 200 is assigned a resource for transmission of RNG-REQ message through Fast_Ranging_IE included in UL_MAP at an appointed time after an expected action time (step 209), and transmits RNG-REQ message to a target base station 230 using the assigned resource (step 210). At this point, the target base station 230 transmits an RNG-RSP message including CID update information to the terminal 200 in response thereto (step 211), whereby data transmission/reception between the terminal 200 and the target base station 230 becomes possible (step 212). The method using the Fast_Ranging_IE of FIG. 2 has an advantage that a time taken until data transmission/reception is relatively short compared to the method using the ranging code of FIG. 1 because step of transmitting the ranging code and step of receiving RNG-RSP message, which is a response thereto, are omitted.
In other words, the handover ranging method using the ranging code requires four times of message exchange procedures until transmission/reception of data. That is, it is not until RNG-REQ and RNG-RSP procedures are performed after a resource for RNG-REQ is assigned through handover ranging that user data can be transmitted and received. Therefore, a service delay time for the user is lengthened. On the other hand, the handover ranging method using Fast_Ranging_IE can transmit an RNG-REQ message without competition using an assigned resource, and thus has a simple procedure compared to the handover ranging method using the ranging code, so that a delay time until data transmission/reception is short. However, the handover ranging method using Fast_Ranging_IE has a disadvantage of high probability of failure. Therefore, the handover ranging method using Fast_Ranging_IE has a possibility that a delay time may be lengthened because a connection should be made again through handover ranging in the case where a handover fails.